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PubMed2005

Screw-sense inversion characteristic of alpha-helical poly(beta-p-chlorobenzyl L-aspartate) and comparison with other related polyaspartates.

This is one of a series of studies on the reversal of the helix sense of polyaspartates originated from the pioneering work of Goodman and his associates in 1960s. Poly(beta-p-chlorobenzyl L-aspartate) (PClBLA) is one of the well-studied polyaspartate derivatives in both solution and the solid state. The chemical structure of PClBLA differs from those of poly(beta-benzyl L-aspartate) (PBLA) and poly(beta-phenethyl L-aspartate) (PPLA) only at the terminal of the relatively long side chain. PBLA takes a left-handed form (L) in conventional helicoidal solvents and does not exhibit any screw-sense inversion. In contrast to PBLA, both PClBLA and PPLA form a right-handed helix (R) in chlorinated alkane solvents and exhibits a reversal of alpha-helix sense at higher temperatures. Yet the transition behaviors in the presence of denaturant acid are quite different between these two polymers. While PPLA exhibits transitions such as R --> L --> coil by lowering temperature, PClBLA directly goes into the coil state without showing the reentrant L form. The cause of these phenomenological differences among these polymers has been investigated by constructing the phase diagram.

Benzyl Compounds↗
PubMed1978

Gas phase carboxylate anions fron p-nitrobenzyl esters of fatty acids.

Gas phase carboxylate anions have been efficiently generated from the p-nitrobenzyl esters of nine fatty acids ranging in size from C6 to C22. The process involves the dissociative resonance capture of 2.9 e V electrons by the derivatives. The procedure is specific and gives the carboxylate anions as the base peak in all but two of the derivative. The technique gives extremely simple spectra, with little additional fragmentation and no rearrangement ions.

Benzyl Compounds↗
PubMed1979

Quantitative analysis of tiamenidine in human plasma by gas chromatography mass spectrometry of a dibenzyl derivative.

A gas chromatography mass spectrometry method has been developed and evaluated for the quantitative analysis of tiamenidine in plasma. Tiamenidine and internal standard are extracted from basified plasma, converted to dibenzyl derivatives by reaction with benzyl bromide and potassium t-butoxide in the presence of 18-crown-6 ether prior to analysis by selected ion monitoring. The method can be used over the range 0.2--10 ng ml-1 with a coefficient of variation of better than 20% at 1 ng ml-1.

Alkylation↗
PubMed1989

Deuterium-labeled 3-nitrobenzyl alcohol as a matrix for fast atom bombardment mass spectrometry.

The utility of deuterium-labeled 3-nitrobenzyl alcohol (DNBA) as a fast atom bombardment (FAB) matrix for establishing the number of exchangeable hydrogens present in a molecule is illustrated by the analysis of five selected antitumor agents. A method for the simple preparation of this labeled matrix is described. The use of DNBA may be of value for samples which provide no FAB spectra when deuterium-labeled glycerol is used as a matrix.

Benzyl Alcohols↗
PubMed1990

Reduction processes in fast atom bombardment mass spectrometry: interdependence of analyte and matrix redox potentials.

The reduction of selected redox indicator dyes during fast atom bombardment mass spectrometry was investigated in several different matrices. Reduction is indicated by the incorporation of one or more hydrogens into the cation of the dye. The extent of reduction of the analyte can be related to the E 1/2 value of the reduction potential of the dye. Selection of matrix also influences the reduction, the reducing power decreasing in the order glycerol greater than sulfolane greater than thioglycerol greater than hydroxyethyl disulfide greater than nitrobenzyl alcohol. Evidence is given suggesting that reduction is initiated by exposure of the analyte-matrix mixture to atom bombardment. It is shown that reduction can be controlled by the addition of metal ions with a very low reduction potential to the matrix to act as electron scavengers.

Azo Compounds↗
PubMed2004

Triisobutylaluminium and diisobutylaluminium hydride as molecular scalpels: the regioselective stripping of perbenzylated sugars and cyclodextrins.

To explain the remarkable regioselective de-O-benzylating properties of diisobutylaluminium hydride (DIBAL-H) and triisobutylaluminium (TIBAL) towards polybenzylated sugars or cyclodextrins, we propose a plausible mechanistic rationale critically involving the kinetic formation of a product-generating 2:1 Al-benzylated sugar complex. For the reaction to occur, one pair of adjacent oxygen atoms should first be able to form a chelation complex with the first equivalent of aluminium reagent, either a highly fluxional complex with tetracoordinate aluminium species or a pentacoordinate one. The second equivalent then induces the regioselectivity of the de-O-alkylation by coordinating preferentially to one of the oxygen atoms of the selected pair.

Aluminum↗
PubMed2005

Transition metal complexes coordinated by an NAD(P)H model compound and their enhanced hydride-donating abilities in the presence of a base.

The ruthenium(II) and rhenium(I) complexes containing an NAD(P)H model compound, 1-benzyl-1,4-dihydronicotinamide (BNAH), as ligand, [Ru(tpy)(bpy)(BNAH)]2+ (1 a) and [Re(bpy)(CO)3(BNAH)]+ (1 b), were quantitatively produced by the reaction of the corresponding metal hydrido complexes with BNA(+) (1-benzylnicotinamidium cation). In the presence of base with pK(a) = 8.9, 1 a and 1 b have much greater reducing power than "free" BNAH. The oxidation potentials of 1 a in the absence and the presence of triethylamine were 0.55 V and -0.04 V, respectively, versus Ag/AgNO(3), whereas that of "free" BNAH was 0.30 V. Spectroscopic results clearly showed that the base extracts a proton from the carbamoyl group on 1 a and 1 b to give the deprotonated BNAH coordinating to the transition-metal complexes [Ru(tpy)(bpy)(BNAH-H+)]+ (3 a) and [Re(bpy)(CO)3(BNAH-H+)] (3 b); this deprotonation underlies the enhancement in reducing ability. The deprotonated forms 3 a and 3 b can efficiently reduce other NAD(P) models to give the corresponding 1,4-dihydro form, resulting in the deprotonated BNA+ being coordinated to the metal complexes [Ru(tpy)(bpy)(BNA(+)-H+)]2+ (2 a) and [Re(bpy)(CO)3(BNA+-H+)]+ (2 b); "free" BNAH and the protonated adducts 1 a and 1 b cannot act in this way. X-ray crystallography was performed on the PF6- salt of 2 a, and showed that the deprotonated nitrogen atom on the carbamoyl group coordinates to the ruthenium(II) metal center with a bond length of 2.086(3) Angstroms. Infrared spectral data suggested that the deprotonated carbamoyl group on the reduced forms 3 a and 3 b is converted to the imido group, and that the oxygen atom coordinates to the metal center.

Hydrogen-Ion Concentration↗
PubMed2006

A facile method for oxidation of primary alcohols to carboxylic acids and its application in glycosaminoglycan syntheses.

A convenient two-step, one-pot procedure was developed for the conversion of primary alcohols to carboxylic acids. The alcohol was first treated with NaOCl and TEMPO under phase-transfer conditions, followed by NaClO2 oxidation in one pot. This reaction is applicable to a wide range of alcohols and the mild reaction conditions are compatible with many sensitive functional groups, including electron-rich aromatic rings, acid-labile isopropylidene ketal and glycosidic linkages, and oxidation-prone thioacetal, p-methoxybenzyl, and allyl moieties. Several glycosaminoglycans such as heparin, chondroitin, and hyaluronic acid oligosaccharides have been synthesized in high yields by using this new oxidation protocol.

Alcohols↗
PubMed1975

Pharmacokinetics of bethanidine in hypertensive patients.

The pharmacolinetics of bethanidine-14C was studied in three hypertensive patients. A 25-MG DOSE OF BETHANIDINE-14 C hemisulfate was administered intravenously. Plasma levels of drug were measured over the first 6 hr. In 3 to 4 days, 89% to 94% of the dose was excreted in the urine. Thin-layer chromatography (TLC) and isotope dilution analysis of the urine samples indicated that only intact bethanidine was excreted. Plasma level and urinary excretion rate profiles had miltiphasic characteristics. Estimated half-lives of the terminal phase ranged from 7 to 11 hr. Average renal clearance over the initial 6 hr approached renal plasma flow. In 2 of the patients, renal clearance between 2 and 4 hr after administration was reduced to one-helf that observed during the initial 2-hr period. After single oral administration of a 25-mg dose of bethanidine-14C hemisulfate, 48% of 61% was excreted in urine and 15% to 48% in feces. Peak urinary excretion rates were reached 6 hr following administration. The urinary excretion kinetics of bethanidine during and after repetive oral dosing was also studied. A 25-mg dose was dividied into 12 to 16 equal doses and administered avery 6 hr. A larger fraction of the cumulative dose was recovered in the urine (72% to 74%) than after the single dose, suggesting higher availability at the lower dose. Steady-state urinary excretion rates were achieved in 4 to 7 doses. The steady-state urinary excretion levels were consistent with pharmacolinetic predictions based on single oral dose data. When 2 of the patients were given imipramine for 2 days prior to an oral 25-mg dose of bethanidine-14C hemisulfate, the terminal half-lives of the urinary excretion rate profiles were shorter than those in the same patients not given imipramine.

Administration, Oral↗